Autism spectrum disorders (ASD) affect neurodevelopment and give rise to social and communication impairments accompanied by stereotyped patterns of behavior and interest. ASD affects 1/110 children and is a public health concern. ASD is not usually diagnosed until after the third birthday. Earlier detection is needed to provide access to intervention as early in life as possible because early intervention optimizes outcomes. The purpose of the proposed study is to identify very early indicators of ASD, beginning in infancy. Specific aims are to: (1) define markers of disrupted neurobehavioral development that appear between 6 and 14 months of age, before clinical signs of ASD; (2) translate ASD clinical indicators identified in the second year of life into early ASD diagnostic criteria applicable to 1- and 2-year olds; and (3) define protective factors associated with outcomes in sibs-A to inform treatment approaches. These aims will be achieved through the use of a prospective, longitudinal research design where assessments occur at regular intervals between 6 and 36 months. Participants will include younger siblings of children with autism (sibs-A) who are at high genetic risk for ASD as well as language and social delays; preterm infants, who are at high risk for developmental delays but lower risk for ASD than sibs-A; and younger siblings of children with typical development who are deemed to be at low risk for developing ASD. At 36 months, children will receive confirmatory outcome classification into one of three groups: ASD; Non-ASD Delay; and No Delays. Measures administered at each age assess visual attention to social and non-social stimuli, motor and communication development, object exploration, understanding of others' intended actions, and ASD symptoms. Statistical analyses will include ANOVA, Pearson's correlation, Classification and Regression Trees, regression models, and propensity score methods. High-impact findings are anticipated. Based on discoveries from this study about the nature of very early disruptions in development, new ASD screeners for infants and toddlers may be developed that are more sensitive and specific than existing tools. In addition, proposals for the content and instructional methods used in very early interventions will be made. Such interventions may prevent aspects of the ASD phenotype from emerging, or dampen the severity of expression of the disorder.